1. Field of the Invention
The present invention relates to a Micro Electro-Mechanical System (MEMS) structure, and in particular to a microphone structure.
2. The Prior Arts
In the past thirty years, the complementary metal oxide semiconductor (CMOS) has been used extensively in the manufacturing of Integrated Circuits (IC). The development and innovation of IC have progressed by leaps and bounds, due to huge amount of research manpower and investment put in, to raise significantly its reliability and yields; meanwhile, its production cost is reduced drastically. Presently, that technology has reached a mature and stable level, such that for the continued development of the semiconductor, in addition to keeping up the present trend of technical development, it is essential to achieve breakthrough to provide special production process, and enhance system integration of high concentration.
In this respect, the Micro Electro-Mechanical System (MEMS) is a new processing technology completely different from the convention technology. It mainly utilizes the semiconductor technology to produce MEMS structure; meanwhile it is capable of making products having electronic and mechanical functions. As such, it has the advantages of batch processing, miniaturization, and high performance, and is very suitable for use in Production Industries requiring mass production at reduced cost. Therefore, for this stable and progressing CMOS technology, the integration of MEMS and circuitry can be a better approach to achieve system integration.
For the processing of most of the MEMS elements, poly-silicon is utilized to make active elements, such that it utilizes one or more oxides as the release layer, the silicon nitride as the isolation layer, and metal layer as a reflector and internal connection. In processing the MEMS elements, it could encounter an especially difficult release problem, such that in this process, a silicon oxide sacrifice layer is dissolved, and a gap thus created is to separate various elements. In this respect, the MEMS elements, including the electrostatic suspension arms, the deflection mirrors, and the torsion regulator are released through dissolving the sacrifice layer by means of the wet chemical process. In general, that process is performed on a single piece of MEMS circuit chip, rather on a whole wafer. At this time, the static friction is liable to cause decrease of yield. The static friction refers to two adjacent surfaces stick to each other, as caused by the capillary forces produced by drying up the liquid between two micro-structures, thus leading to decrease of yield. Most of the MEMS elements are made through using oxide sacrifice layers. Usually, a water containing hydrofluoric acid is used to dissolve an oxide sacrifice layer to achieve release. In another approach, a hydrofluoric acid vapor is used to release MEMS elements having oxide sacrifice layer.
In a thesis of Stanford University, “Wafer Scale Encapsulation Of Large Lateral Deflection MEMS Structure”, the MEMS element is produced by first performing Deep Reactive-Ion Etching of a silicon-on-insulator (SOI) substrate. Next, grow a layer of silicon dioxide thereon, and then planarize its surface. Subsequently, form a first epitaxy layer, and then perform deep reactive-ion etching to remove a part of the silicon layer. Finally, grow a second epitaxy layer to seal off the etched holes on the first epitaxy layer, to form an electrode serving as a connection pad. In addition, in U.S. Pat. No. 7,621,183, another MEMS element manufacturing method is disclosed. Wherein, firstly, form an oxide layer on a cap wafer, then form a balance structure and a germanium layer on a gyroscope wafer, to connect the gyroscope wafer onto the cap wafer. Finally, connect a reference wafer electrically to the germanium layer, to fix it on the gyroscope wafer. From the descriptions above it can be known that, the former cited case utilizes the epitaxy technology requiring high price metal; while for the latter cited case, its production process is rather complicated.
In the traditional technology, a microphone structure fabricated by MEMs technology comprises two electrode plates, and a insulation material is arranged between the electrode plates. However, the distance between the electrode plates is fixed. If the developer wants to greatly increase the capacitance of the microphone structure, the areas of the electrode plates are greatly enlarged, which greatly increases the fabrication cost. Besides, releasing the insulation material is a complicated fabrication process.
In view of the problems and shortcomings of the prior art, the present invention provides a microphone structure, that is simple in implementation, to overcome the deficiency and drawback of the prior art.